Semiconductor light-emitting devices including light emitting diodes (LEDs), resonant cavity light emitting diodes (RCLEDs), vertical cavity laser diodes (VCSELs), and edge emitting lasers are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness light emitting devices capable of operation across the visible spectrum include Group semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. Typically, III-nitride light emitting devices are fabricated by epitaxially growing a stack of semiconductor layers of different compositions and dopant concentrations on a suitable substrate by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. The stack often includes one or more n-type layers doped with, for example, Si, formed over the substrate, one or more light emitting layers in an active region formed over the n-type layer or layers, and one or more p-type layers doped with, for example, Mg, formed over the active region. Electrical contacts are formed on the n- and p-type regions. III-nitride devices are often formed as inverted or flip chip devices, where both the n- and p-contacts formed on the same side of the semiconductor structure, and light is extracted from the side of the semiconductor structure opposite the contacts.
III-nitride substrates are generally expensive and not widely available, thus III-nitride devices are often grown on sapphire or SiC substrates. Such non-III-nitride substrates are less than optimal because sapphire and SiC have different lattice constants than the III-nitride layers grown on them, causing strain and crystal defects in the III-nitride device layers, which can cause poor performance and reliability problems.
A composite substrate, which may have a lattice constant that more closely matches the lattice constant in the light emitting layer in the device, is shown in FIG. 1 and described in US 2007/0072324, which is incorporated herein by reference. Substrate 10 includes a host substrate 12, a seed layer 16, and a bonding layer 14 that bonds host 12 to seed 16. Each of the layers in substrate 10 are formed from materials that can withstand the processing conditions required to grow the semiconductor layers in the device. Device layers 18 are grown on seed layer 16. Bonding layer 14 may be a release layer formed of a material that can be etched by an etch that does not attack device layers 18, thereby releasing device layers 18 and seed layer 16 from host substrate 12. The composition of the layer adjacent to seed layer 16 may be chosen for its lattice constant or other properties, and/or for its ability to nucleate on the material of seed layer 16. In one example, host 12 is sapphire and seed layer 16 is InGaN.